Perpustakaan Digital ITB

For the past fifty years, the northern Songliao Basin in China was thought to contain Lower Cretaceous coal-measure and gas-generating shale. Oil-type shales did not receive much attention until oil exploration activities made a breakthrough in the Southern Shuangcheng (SSC) depression. In this study, the geochemical characteristics of the newly discovered K1yc4 (fourth member of the Lower Cretaceous YingCheng Formation) oil-type shale in the SSC depression were analyzed, including the organic matter abundance, organic matter type, thermal maturity, hydrocarbon potential, and sedimentary environment. A total of 393 core samples were collected, and Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS), organic petrography, carbon isotope analysis, and constant and trace element analyses were conducted. The K1yc4 shale was determined to have a good to excellent hydrocarbon potential with high TOC values (mostly 2.0–4.0%). The organic matter was mainly from lower hydrobiont, algae, bacteria, and lesser amounts of terrestrial plants. The organic matter is mainly type I to type II based on the hydrogen index (HI) (average HI = 193.36 mg HC/g TOC), the carbon isotopes of the kerogen (mostly ?26 to ?30‰), and the organic petrography (average sapropelinite + liptinite of 55.0 vol%). The maturity parameters, i.e., Tmax (mean 452 °C) and vitrinite reflectance (0.8–1.1%), suggest that the shale is in the mature thermal evolution stage. The shale's maximum hydrocarbon-generating intensity is up to 713.29 × 104/km2, and its hydrocarbon-expelling intensity is stable at 116.63 × 104/km2. The constant and trace element analyses (Sr, Ba, V, Ni, Cr, Ti) revealed that the organic matter deposition and preservation occurred in a fresh-brackish and dysoxic-anoxic water environment. Volcanic eruptions probably played a vital role in the enrichment of the organic matter in the oil-type shale. In addition, the results of this study provide evidence that the Lower Cretaceous strata in the northern Songliao Basin meet the geological conditions of oil-prone shale, which expands the petroleum exploration horizon in this region.